How close to interstellar space travel could humans get in the near future?

Question

I had in mind a sci-fi setting where humans have begun terraforming nearby planets without the benefit of faster-than-light travel. For example, Wikipedia lists a handful of terrestrial planets within 15 light years. What I have in mind is that humans have the capability to send unmanned probes to those planets after about a 50-year voyage, and manned probes after slightly longer.

I had hoped to set these stories in the last 22nd century, roughly 150 years from today. I could go as far as 250 years in the future, but I don't want the space-farers to be too far removed from modern Earth culture.

• How could I make it plausible that by that time, human beings have created spacecraft of those speeds?
• What are the major barriers mankind would have to overcome to reach those speeds?

Answer 1

Let's do a calculation or two, shall we? (Note: All those wishing to skip the boring math can just read the lower portion of this answer.)

Finding the speed you're going for: $$v=\frac{15 \text { light-years}}{50 \text { years}} \times \frac{10,000,000,000,000,000 \text { meters}}{1 \text { light-year}} \times \frac{1 \text { year}}{31,500,000 \text { seconds}}= 95238095.24 \text { m/s}$$

Finding the energy needed to get to that speed: $$E_k=\frac{mc^2}{\sqrt{1-(v/ c)^2}}-mc^2$$ Assuming a mass of about $150,000$ tonnes (taking the Project Daedalus figure and tripling it), $$E_k=\frac{(150,000,000)(300,000,000)^2}{\sqrt{1-((95238095.24)/(300,000,000))^2}}-(150,000,000)(300,000,000)^2$$ $$=7.36 \times 10^{24} \text { Joules}$$

Can we do it? . . .

According to Wikipedia

In 2011, total world energy consumption was 549 exajoules

Multiply that by 50 years, and we still don't have enough.

I suppose we could use the Kardashev scale to figure out when humanity will have energy at that rate. Using this formula: $$K=\frac{\log_{10}P-6}{10}$$ and substituting in humanity's energy usage per year, we could calculate just when we'll reach that point. Just use $$P= \text { Joules/year}=2.34 \times 10^{16} \text { Watts}$$ $$K=\frac{\log_{10}7.36 \times 10^{24}-6}{10}$$

and we find we should be a Type 1.03692 civilization. Given that we're currently a Type 0.7 civilization, we should be at that point in about 100 to 200 years. Note, though, that this figure is for a ship using all of humanity's total energy, which is unrealistic. However, it shows that we won't be able to harness that energy at all for a while.

I would also be obliged if someone could check my calculations. I used this logarithm calculator, in case anyone wants to use it. Conversion ratios can be found easily online.

---

What are the major barriers mankind would have to overcome to reach those speeds?

If we're talking purely about travel at this speed, i.e. assuming we've figured out everything else, the main problem is propulsion. As the above result showed (for those who skipped it, you just have to know that we'd need a lot of power to reach these speeds.), it's not easy to travel fast in space. Here are some options:

• Bussard Ramjet - Take in hydrogen gas floating in interstellar space, compress it, begin fusion, and convert that into thrust. Obstacles: It has to be really big, and you have to be sure you have hydrogen in space. Oh, and you have to master fusion.

• Nuclear Pulse - Chuck nuclear bombs out the back of the spacecraft, and ride the shockwaves. This was used in the hypothetical Project Daedalus and the equally hypothetical Project Orion. Neither idea got off the ground (pun definitely intended). Obstacles: You have to make loads of nuclear bombs. Also, you have to find a nation that's willing to sacrifice a bunch of land for the launch pad, because that launch pad and the surrounding area will be completely destroyed.

  Alternatively, you could just use this idea in space, as Lohoris suggested.

• Solar Sail - Ride the radiation pressure from a star using a sail. Obstacles: You've got to make and deploy an enormous sail kilometers wide. Also, the acceleration is incredibly slow.

• Antimatter Rocket - I've saved the best for last. Put together matter and antimatter and harness the energy from the annihilation. Obstacles: Very expensive, because antimatter is expensive. Also, you'll have to make quite a lot of it at first, although you can start to coast at some point. You have to be very delicate, though. Veeeeery delicate.

Answer 2

Most likely space colonization will not go as you envisage.

Actually there could be hundreds and thousands of planets within 1 light-year from the Sun. If there are no stars that close does not necessary mean there are no planets, both wandering and orbiting the Sun. Statistics predict that there are much more wandering planets in the galaxy than stars.

As such, future people may not think it is necessary to travel to other stars before they colonized at least the solar system. As they explore it they find more and more celestial bodies suitable for colonization, gradually approaching other stars.

Answer 3

Tough but possible.

$15ly$ in $50$ years would require $0.3$ times the speed of light. To reach this speed, we could accelerate with $4g\approx 40m/s^2$ for 26 days (or 51 days with $20m/s^2$ or 13 with $80m/s^2$ .. the last one would be very unpleasent for the passengers).

How much energy would it take? Lets stick at $150000$ tons. So its $E_1=\frac{1}{2}mv^2=\frac{1}{2}m(0.3c)^2\approx6e23\,\text{Joule}$. If we use relativistic formulas, we would need $6.5e23 \text{Joule}$. Close enough for our estimates. But wait! Now we reach the target system but still travel with a notable fraction of the speed of light. We need to get rid of this velocity and need the same amount of energy again. This sums up to $E=E_1\times 2=1.2e24 \text{Joule}$. That's nearly 8 tons of antimatter by the way.

How do we get the energy? Lets try solar power. The solar constant is $E_0=1367\frac{W}{m^2}$ near earth. This constant is proportional to the inverse of the distance to sun squared. At $0.1AE$ (about one third of the distance from sun to mercury), its $E_{0}^\text{Mercury}=137\frac{kW}{m^2}$. To get enough energy in one year, we need to cover an area of $\frac{E}{E_{0}^\text{Mercury}}\times \text{seconds per year}\approx 7e12m^2$ (that's arround $2700km \times 27000km$ - tough but possible). You have to scale it up for general loss due conversion and inefficencies though. There are other ways like fusion reactors, magic cold fusion, etc. Your choice.

Now the hardest part: how to convert the energy in velocity?

• Solar panels? The acceleration is painfully slow in our solar system and goes down with the distance squared (double the distance, acceleration goes down by a factor of 4).
• You don't want to use an FTL so you can't invent a classical warp drive. Maybe you can invent something like a slower-than-light-warp but this would render all calculations useless as there is no knowledge about it. If you want to stick at hard SciFi without warp, you need something to exhaust to push you forward.
• Classical propolusion drives are out of question too. Take a look at the rocket equation. The Ariane 5 rocket have a specific impulse of $440s$. Putting in our numbers, we come up with a mass around $e^{20850}kg$ - more than double precision can handle. You need something to push away with a notable fraction of $c$.
• The idea with nuclear bombs (antimatter?) driving you forward is tempting. But explosions are short bursts and we need many of them over a big time (remember the $4g$ over 26 days?). This is a big challenge for the structure of tyour space ship.
• I think you have to invent something new like a large scale particel accelerator shooting matter out with the speed of light. You still need quite an amount of this particels ... for the needed $\Delta v=0.6c$ $0.8$ times your ship's mass - $123'000t$. Remeber to scale up your solar panels.

Every SciFi I know reaching out for other solar systems - even the hard ones - have some magical device/ thruster without real science behind them (warp drive, warp tunnels, instant teleportation over light years, ...). I don't think you can get around it.

Now the social component. Try cryo sleep chambers, thats the easiest (I'm more a technical guy).

Answer 4

Ok, so the best and most enlightened answers here have considered the amount of energy required to travel these distances.

If you want to shortcut over the hundreds of years it will take us to get to this point you could try the following:

• External Influence

Somebody like the Vulcans visit us from another planet and share advanced space flight technology. Or perhaps we steal the technology after they visit.

• Gift from Space

Something like a meteor strikes the Earth carrying large quantities of some new type of fuel compound which is more effective than anything we have on Earth.

• Time Travel

People from the far future come to our time to share with us the advanced technology required for space flight.

• Messages from Space

An Alien race sends us the designs for building interstellar craft.

• Unknown Technology

At some point in the not too distant future a breakthrough is made that changes our understanding of Space and Time, allowing us to get to this level of technology faster

Answer 5

Yes, based on our calculations for energy requirements, which is right on, and the Kardashev scale 300 to 350 years would be a good guess.

This would be for a sublight starship using 7 x10 ^24 joules... likely fusion powered. Still very "pre-warp", as warp drive would likely require Kardashev mid level two civilization...

Though as Nick R stated, new "multiple" discoveries resulting in paradigm shifts could accelerate that Daedalus type ship by perhaps shaving 100 years off that 300 - 350 year estimate...

Answer 6

With an electricmagnetic thruster like VASIMIR and a nice small fission reactor like SNAP, you're more like 50 years from starting an unmanned probe. OK, you still need a lightweight, space-rated >200W generator.

A manned probe? Think again. And forget about transporting terraforming equipment. A probe with ten times improved exhaust velocity from VASIMIR (500km/s from 50km/s, which is not too ambitious for 50yrs imo) needs to have less than 1% payload ratio to get to 0.01 c.

With 1000km/s,things would look like they become practically useful, like having enough fuel to slow down after the cruise (Initial payload ratio for acceleration is square root of absolute payload ratio). But still more like 250 years travel time. You would need a fusion reactor here, admittedly. Apart from it's far better efficiency (energy/kg fuel) it has the added benefit that you might use the fusion product as additional propulsion medium.

Useful: http://www.wolframalpha.com/input/?i=rocket+speed

Answer 7

One unified field theory suggests 11 dimensions. If true, maybe one could traverse a 3-dimentional distance by passing through one or more higher dimensions. Folding space, or folding yourself to slip between dimensions. You might need a gate to enable a return trip unless the device was small enough to take with you.

Answer 8

NASA plans to have their Warp vessel built by 2100 so it is unlikely we would try any interstellar travel until proves to be unfeasible. Given that as a starter point we should expect failure between now and then in your universe.

A Mars mission is planed in the mid 2030s and might get pushed back by as much as a decade. You can then expect at least a decade or two dealing with Mars.

This puts you at 2050 before we get to large scale warp trials. I'd expect at least 10-30 years of trials and this gives us a 2080 date of when probes and such are going to start to be planned.

within a decade or 2 of this we'll launch probes which will be equipped with ion thrusters or solar sails or something and there are a lot of stars within 50-100 light years and each take more than 8 years to get to and 4 to communicate back. 12 years is the minimum for that which means you're now up 2112 before we get info back on any star. Assuming a perfect hit we then have a decade or 2 to plane the mission and more than 8 years to get to the planet, because the ship is going to probably have to go slower, but forgetting that, the minimum time year that we'll land on another planet to colonize it is 2140 if we don't have warp of any kind.

Ironically Warp would probably push this back because we then wouldn't rush to colonize because time wouldn't really be as big an issue and you run into that thing about speed of ships would constantly outpace each other so it's best to build a significantly advanced warp ship where the outpaced development wouldn't matter.

A more realistic timeline would be 2200s or 2300s simply to get all the data back and do a lot of testing of close range vessels for longer periods to make sure they get to where they're meant to be going and also transporting building materials.